I buy formula truck for a friend now, will he get the formula truck dlc for automobilista later if he buys automobilista after trying formula truck?
I'm not so sure right now, but based on what we have said previously on this topic: Reiza Membership packages now in offer - what they are and why join If he already owns F-Truck 2013 and Automobilista by the time that the DLC is released for AMS, then yes, he will get it with no costs. We will have more details available to the public by the time of release of this DLC comes.
Yes, if you own Formula Truck on Steam (be it by your own purschase or from a gift), you will have the AMS Truck DLC available for free (so long as you also own AMS obviously).